That does mean that the app doesn't get a price from its database.

It's not possible to check why because you didn't provide a travel date.

But the fee is 30 € 2nd class or 38 € 1st class for London to Paris and 18 € for Paris to Saarbrücken (per person); for Saarbrücken to Landstuhl there's no reservation needed / possible. 

Possible to book here (subject to availability): https://www.raileurope.com/ (desktop version only).

Thank you for the reply and price list, that's so kind of you! My travel date is 13 May 2024, could it be too far out to book a reservation? 

Definitely not too early. You should book Eurostar as soon as your dates are set as there’s a passholder quota (pretty much unique in Europe). 13th May is also right after a busy holiday weekend (Ascension).

Have a look at this general guide : https://www.seat61.com/interrail-and-eurail-reservations.htm

Rail Europe works well : no fees, clear overview,… I’ve used it myself :)

No, it's already a little bit late. London to Paris is already sold out in 1st class; just 2nd class has still availability for the chosen train.

Paris to Saarbrücken has availability. To book this one with raileurope.com (one by one), you need to choose Kaiserslautern instead of Saarbrücken as destination (but it's no problem to get off in Saarbrücken).

N/A just means that the app could not retrieve a price from its online database for some reason. Maybe because you don't have an internet connection. There's no relation at all with seat availability on the train: the app doesn't know that. The prices it shows can also be wrong so I tend to ignore them anyway.